Some structures are designed with a higher than usual level of safety against partial or complete failure due to their functions and the disastrous consequences of their structural disintegration. However, many of such structures have been designed and built without considering some of the very high impactive or impulsive loads on the assumption that the probabilities of occurrence of such loads are extremely low. As time elapses, the changing circumstances of the world may render this probabilistic assessment obsolete and the probabilities of occurrence of such hazards become non-negligible. As an example of having structures subjected to unexpected hazards is the terrorist attack of Sep. 11, 2001, where three aircrafts crashed upon the two towers of the World Trade Center and the Pentagon building in the United States of America. Many other important structures such as: nuclear reactor containments, nuclear waste storages, large oil or natural gas reservoirs, large chemical containers, ammunition storages and military installations, could be threatened in the future by similar attacks or by accidents or in case of war.
Many of such hardened and rigid structures have reinforced concrete outside walls that may—in some buildings—exceed 2.0 meters in thickness. However, the thickness is usually less when the wall is made of pre-stressed concrete. It is also common to have the structure lined with a layer of steel or a non-metallic material. Moreover, reinforced concrete structures which are partially or completely buried under compacted layers of soil are common, especially, in military installations. Furthermore, it is also a common concept of design to have a cluster of buildings where the building which is required to be the most protected is surrounded by the others.
The common character of most of the above mentioned concepts is the very high rigidity of the outside walls of the structure, which represents a strong shield that is hard to penetrate by hard or soft missiles. However, the challenges represented by a crash of a large civilian air craft or a smart missile which could penetrate thick walls of reinforced concrete, require innovative designs that offer more protection for such important structures and to increase their capabilities to withstand very high impactive and impulsive loads.